1. Field of the Invention
The present invention relates to inverter systems, which include a PWM-type (pulse-width-modulation type) electric power converter and an ac rotating machine connected to the electric power converter through a three-phase cable, for reducing surge voltage generated in the ac rotating machine when the systems operate. Moreover, the present invention relates to ac rotating machines and electric power converters that can reduce the surge voltage.
2. Description of the Related Art
In a PWM-type electric power converter, output voltage is adjusted by on-off controlling switching devices of an inverter with a high-frequency carrier signal. In a three-phase cable connecting this electric power converter with an ac rotating machine (motor and dynamo), a resistive component R and an inductive component L exist corresponding to the cable length, and capacitance C also exists between lines of the cable, and between each of the lines and ground. As a result, when rapidly varying voltage is outputted from the electric power converter to the three-phase cable, by resonance due to R, L and C the over-voltage is applied to terminals on the ac rotating machine. Therefore, in order to reduce the over-voltage at the terminals of the ac rotating machine, an over-voltage reduction means has conventionally been provided together with the terminals (for example, refer to Patent document 1).
According to Patent document 1, in an input/output non-insulating electric-power converter having a rectifier for converting ac to dc and an inverter for converting the dc to ac, an over-voltage reduction means is disclosed, in which over-voltage at an electric-power-supply terminal of the rotating machine is reduced by a method of connecting the neutral points of capacitors in an ac output filter including the capacitors and reactors with a dc circuit on the electric-power converter.
Moreover, in a driver having an electric power converter and an ac rotating machine, in order to prevent common-mode noise, which is generated in the electric power converter, from affecting external equipment, a coaxial-cable core line has been conventionally used for connecting the output terminal of the electric power converter with the input terminal of the ac rotating machine, and each of the outer conductor ends of the coaxial cable has been connected to the frame (ground terminal) of the electric power converter and to the frame (ground terminal) of the ac rotating machine (for example, refer to Patent document 2). The purpose of the driver as described above is to prevent the common-mode noise, but not to prevent the surge voltage inside the rotating machine; however, using three single-core coaxial cables or a three-core coaxial cable can resultantly reduce the over voltage at the ac rotating-machine.
[Patent document 1] Japanese Laid-Open Patent Publication 294,381/1997
[Patent document 2] Japanese Laid-Open Patent Publication 135,681/1998